NMR Study of Nuclear Spin Polarization During Chemical Reactions with Ortho Hydrogen

Polarization signals in NMR spectra do not necessarily point to reactions via radicals. This is confirmed by hydrogenations (a) with enriched ortho and para hydrogen, in which polarization signals were observed in each case, albeit with different intensity and reversed sequence of emission and absorption. the intensity ratio can be correlated with the ratio of the ortho and para hydrogen, and thus it is unequivocally confirmed that the nuclear spin states of the H ₂ molecules are responsible for the non-equilibrium occupation of the product nuclear spin level.